The present invention relates to the preparation of orthopaedic mixtures and compounds, such as bone cement. More specifically, the invention concerns systems and methods for identifying the operating state of these compounds, such as the degree of cure of bone cement.
It is necessary in many orthopaedic surgical procedures to employ a cement or grouting type agent, such as for attaching artificial joint implants to bone, repairing or forming joints in bones, or other forms of orthopaedic work. The type of cement used typically depends upon many factors, including the type of implant, the manner of application, the amount of working time required, etc. While many types of bone cement are available, most of the cements used for orthopaedic purposes include a self-curing resin formed from the blending of a wide variety of liquid monomers or co-monomers with powdered polymers or copolymers to form a viscous admixture to be used as the grouting agent. Most bone cements are acrylate-based compositions formed of a liquid component and a powder component. A typical liquid component is a liquid mixture of a monomeric methyl methacrylate. The powder component generally consists of a methylmethacrylate-styrene copolymer. Curing of the liquid-powder composition occurs as the constituents polymerize and cross-link.
The admixture of the powder and liquid components develops a quick setting material. As such, preparation of the cement usually occurs directly within the operating area just prior to use. In particular, a bone cement mixing apparatus is generally utilized to mix the powder and liquid components in the operating area. The resultant admixture is then removed from the mixing apparatus and placed in a cement delivery apparatus for subsequent use by the surgeon. Specifically, the bone cement must generally first be scooped or otherwise removed from the mixing apparatus and thereafter placed in a syringe-type delivery apparatus for use by the surgeon. In other cases, the bone cement components are mixed directly in the delivery apparatus, which eliminates the need to transfer the bone cement from a mixing apparatus to the syringe-type delivery system.
Bone cements typically have setting times between 6½ to 15 minutes. Three operating points characterize the curing of bone cement. The dough time, distinguished qualitatively as the point in time where bone cement no longer sticks to latex gloves, is the first operating point. The dough time, which is measured relative to initial mixing, occurs after the mixing of the bone cement. The dough time is significant as it is identified as the start point of the working time of the admixture. The working time encompasses the amount of time during which the viscosity or flowability of the composition is sufficient to allow introduction of the composition into the surgical or implant site. The end of working time, distinguished qualitatively as the point in time where bone cement no longer sticks to itself, is the second operating point. The end of work time is relative to initial mixing and signifies that the working time has ended and the bone cement should no longer be used in the surgery. The third operating point is the setting time, which is the time relative to initial mixing at which the bone cement admixture hardens or sets sufficiently to hold the prosthesis within its implant site.
Since the overall setting times for most orthopaedic compositions, such as bone cements, are short, it is important that the composition be introduced into the implant site as soon as practicable. For many compositions, the admixture must reach a certain level of cure to have the viscosity, flowability or malleability to allow the material to be properly introduced into the surgical site. For instance, many orthopaedic implants include a stem that is fixed within the intramedullary canal of a bone, such as the femur. As a precursor, bone cement is injected into the prepared intramedullary canal prior to introduction of the implant stem. Optimally, the bone cement flows into porous recesses of the bone to ensure a solid mechanical interlock with the implant. If the bone cement is administered before the appropriate degree of cure, the cement will be too fluid, which may make it difficult to properly administer. Moreover, if the bone cement is too fluid, it may overflow upon application or when the implant stem is introduced.
On the other hand, if the cement has cured too much, it may be too viscous so that it does not fill the bone voids and interstices. This can result in a poor mechanical interlock or interface between implant and bone. Even if the composition is viscous enough for proper application, it may cure in situ to a point that prevents proper positioning and alignment of the implant stem within the bone. An even greater degree of cure will render the bone cement too viscous to be usable.
Bone cement failure is believed to be a primary mechanism for loosening of prosthetic joint components. It can be readily appreciated that it is very desirable to accurately determine the degree of cure or operating state of the composition in the surgical arena so that the composition can be applied at the optimum point in its curing cycle. One ASTM standard relies upon determining the resistance to a plunger pushed into a container of curable material. A similar approach is disclosed in European Patent Publication EP0995981 in which a pre-determined discharge force is applied to a bone cement reservoir and the travel distance of bone cement through a test lead is measured. This testing approach is cumbersome, susceptible to measurement error and not conducive to being repeated as the bone cement cures.
In another approach, an electric current is passed through a bone cement sample. Variation in an electrical property of the bone cement, such as capacitance, is used to determine the amount of cure of the material. (See, e.g., Aesculap German application DE10008481) One drawback with this approach is that it is equipment intensive. Moreover, this approach requires the composition to not only have measurable electrical properties, but also that the electrical properties vary as a function of the cure of the material.
There is a need for a system and method for accurately determining the operating condition or degree of cure of a biological composition, such as bone cement. The need is also felt for such a system that can be used with the composition as it exists in the surgical setting. This need further extends to a reusable system that requires minimal or no redesign of current delivery systems.